Alarm system with latching in PLC Programming - Step-by-Step Execution

Concept Flow - Alarm system with latching

Start

↓

Check Alarm Condition

↓

Is Alarm Active?

No→Keep Alarm Off

Yes↓

Set Alarm Latch ON

↓

Is Reset Activated?

No→Keep Alarm Latched ON

Yes↓

Reset Alarm Latch OFF

↓

End

The system checks if an alarm condition occurs. If yes, it sets a latch to keep the alarm ON until a reset is activated.

Execution Sample

PLC Programming

IF alarm_condition THEN
  alarm_latch := TRUE;
END_IF;

IF reset_button THEN
  alarm_latch := FALSE;
END_IF;

This code sets the alarm latch ON when the alarm condition is true and resets it OFF when the reset button is pressed.

Execution Table

Step	alarm_condition	reset_button	alarm_latch Before	Condition Checked	Action Taken	alarm_latch After
1	FALSE	FALSE	FALSE	alarm_condition = FALSE	No change	FALSE
2	TRUE	FALSE	FALSE	alarm_condition = TRUE	Set alarm_latch = TRUE	TRUE
3	TRUE	FALSE	TRUE	alarm_condition = TRUE	Keep alarm_latch = TRUE	TRUE
4	TRUE	TRUE	TRUE	reset_button = TRUE	Reset alarm_latch = FALSE	FALSE
5	FALSE	FALSE	FALSE	alarm_condition = FALSE	No change	FALSE

💡 Execution stops after processing inputs; alarm latch remains set until reset_button is TRUE.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
alarm_condition	FALSE	FALSE	TRUE	TRUE	TRUE	FALSE
reset_button	FALSE	FALSE	FALSE	FALSE	TRUE	FALSE
alarm_latch	FALSE	FALSE	TRUE	TRUE	FALSE	FALSE

Key Moments - 2 Insights

Why does the alarm stay ON even when the alarm_condition becomes FALSE after being TRUE?

Because the alarm_latch is set to TRUE when alarm_condition is TRUE and stays latched ON until reset_button is pressed, as shown in steps 2 and 3 of the execution_table.

What happens if the reset_button is never pressed?

The alarm_latch remains TRUE (latched ON) indefinitely, regardless of alarm_condition changes, as seen in step 3 where alarm_condition is TRUE but reset_button is FALSE.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. What is the value of alarm_latch after this step?

A FALSE

B TRUE

C UNKNOWN

D Depends on reset_button

💡 Hint

Check the 'alarm_latch After' column at Step 2 in the execution_table.

At which step does the alarm_latch get reset to FALSE?

A Step 3

B Step 5

C Step 4

D Never

💡 Hint

Look for the step where reset_button is TRUE and alarm_latch changes from TRUE to FALSE.

If the reset_button is always FALSE, what will be the alarm_latch value after Step 5?

A TRUE

B FALSE

C Alternates TRUE and FALSE

D Depends on alarm_condition only

💡 Hint

Refer to variable_tracker for alarm_latch values when reset_button remains FALSE.

Concept Snapshot

Alarm system with latching:
- When alarm_condition is TRUE, set alarm_latch ON.
- alarm_latch stays ON even if alarm_condition goes FALSE.
- alarm_latch resets OFF only when reset_button is TRUE.
- Keeps alarm ON until manually reset.
- Useful for persistent alarms needing acknowledgment.

Full Transcript

This visual execution trace shows how an alarm system with latching works in PLC programming. The system checks if an alarm condition is true. If yes, it sets a latch variable to TRUE, keeping the alarm ON. This latch remains ON even if the alarm condition later becomes false. The alarm latch only resets to FALSE when a reset button is pressed. The execution table walks through steps showing inputs and latch state changes. The variable tracker shows how alarm_condition, reset_button, and alarm_latch change over time. Key moments clarify why the alarm stays latched and how reset works. The quiz tests understanding of latch behavior at different steps. This helps beginners see how latching keeps alarms persistent until reset.

Practice

1.

What does a latching alarm system do in a PLC program?

easy

A. Keeps the alarm ON after the trigger until manually reset

B. Turns the alarm ON only while the trigger is active

C. Automatically resets the alarm after a delay

D. Disables the alarm permanently after one trigger

Solution

1. Step 1: Understand alarm behavior

   A latching alarm stays ON even if the trigger stops, unlike a momentary alarm.

2. Step 2: Identify manual reset role

   The alarm stays latched until a manual reset input clears it.

3. Final Answer:

   Keeps the alarm ON after the trigger until manually reset -> Option A

4. Quick Check:

   Latching alarm = stays ON until reset [OK]

Hint: Latching means alarm stays ON until reset manually [OK]

Common Mistakes:

• Confusing latching with momentary alarms
• Thinking alarm resets automatically
• Assuming alarm disables permanently after trigger

2.

Which of the following is the correct ladder logic expression to latch an alarm ALARM when TRIGGER is true, and reset it when RESET is true?

ALARM = (TRIGGER OR ALARM) AND NOT RESET

easy

A. ALARM = (TRIGGER AND ALARM) OR RESET

B. ALARM = (TRIGGER OR ALARM) AND NOT RESET

C. ALARM = TRIGGER AND NOT ALARM AND RESET

D. ALARM = NOT TRIGGER OR ALARM OR RESET

Solution

1. Step 1: Understand latching logic

   The alarm stays ON if triggered or already latched, unless reset.

2. Step 2: Match expression to logic

   ALARM = (TRIGGER OR ALARM) AND NOT RESET matches: ALARM = (TRIGGER OR ALARM) AND NOT RESET.

3. Final Answer:

   ALARM = (TRIGGER OR ALARM) AND NOT RESET -> Option B

4. Quick Check:

   Latching = OR previous alarm + trigger, AND NOT reset [OK]

Hint: Use OR for latch, AND NOT for reset in logic [OK]

Common Mistakes:

• Using AND instead of OR for latch condition
• Forgetting to invert RESET input
• Mixing AND and OR incorrectly

3.

Given the following PLC code snippet, what is the value of ALARM after these inputs?

ALARM = False
TRIGGER = True
RESET = False

ALARM = (TRIGGER or ALARM) and not RESET

TRIGGER = False
ALARM = (TRIGGER or ALARM) and not RESET

RESET = True
ALARM = (TRIGGER or ALARM) and not RESET

What is the final ALARM value?

medium

A. True

B. Undefined

C. Error

D. False

Solution

1. Step 1: Evaluate first assignment

   ALARM = (True or False) and not False = True and True = True

2. Step 2: Evaluate second assignment

   TRIGGER = False, ALARM = (False or True) and not False = True and True = True

3. Step 3: Evaluate third assignment

   RESET = True, ALARM = (False or True) and not True = True and False = False

4. Final Answer:

   False -> Option D

5. Quick Check:

   Reset clears alarm = False [OK]

Hint: Reset input clears alarm regardless of trigger [OK]

Common Mistakes:

• Ignoring reset input effect
• Assuming alarm stays True after reset
• Confusing OR and AND logic

4.

Identify the error in this PLC latching alarm code snippet:

ALARM = TRIGGER AND NOT RESET
ALARM = ALARM OR TRIGGER

medium

A. The alarm will never reset because reset is ignored in second line

B. The alarm resets correctly but never triggers

C. Syntax error due to missing parentheses

D. No error, code works as intended

Solution

1. Step 1: Analyze first line

   ALARM is set only if TRIGGER is true and RESET is false.

2. Step 2: Analyze second line

   ALARM is overwritten by ALARM OR TRIGGER, ignoring RESET input.

3. Step 3: Effect on reset

   Reset input is ignored after first line, so alarm never clears.

4. Final Answer:

   The alarm will never reset because reset is ignored in second line -> Option A

5. Quick Check:

   Reset ignored after first line = alarm stuck ON [OK]

Hint: Reset must be included in all latch logic lines [OK]

Common Mistakes:

• Overwriting alarm without reset condition
• Assuming OR line resets alarm
• Ignoring order of assignments

5.

You want to design a latching alarm that activates when either TEMP_HIGH or PRESSURE_HIGH is true, and resets only when MANUAL_RESET is true. Which expression correctly implements this?

hard

A. ALARM = (TEMP_HIGH AND PRESSURE_HIGH) AND NOT MANUAL_RESET

B. ALARM = (TEMP_HIGH OR PRESSURE_HIGH) AND MANUAL_RESET

C. ALARM = (TEMP_HIGH OR PRESSURE_HIGH OR ALARM) AND NOT MANUAL_RESET

D. ALARM = (TEMP_HIGH AND PRESSURE_HIGH AND ALARM) OR MANUAL_RESET

Solution

1. Step 1: Understand alarm trigger conditions

   Alarm triggers if TEMP_HIGH or PRESSURE_HIGH is true.

2. Step 2: Include latching behavior

   Alarm stays ON if already latched (ALARM) or triggered.

3. Step 3: Reset condition

   Alarm resets only when MANUAL_RESET is true, so use NOT MANUAL_RESET to keep alarm ON.

4. Final Answer:

   ALARM = (TEMP_HIGH OR PRESSURE_HIGH OR ALARM) AND NOT MANUAL_RESET -> Option C

5. Quick Check:

   Latch = OR triggers + alarm, reset = NOT manual reset [OK]

Hint: Use OR for triggers and latch, AND NOT for reset [OK]

Common Mistakes:

• Using AND instead of OR for triggers
• Reset condition inverted incorrectly
• Missing latch condition in expression